The pattern of 5-methycytosine residues in mammalian DNA has recently been found to be crucial to the control of genetic expression. Decreases in DNA 5-methylcytosine content are known to alter the level of differentiation of cells in culture. Thus, changes in DNA 5-methylcytosine patterns may be critical to the process of carcinogenesis. In this regard, we have developed a new method that enables the determination of genomic 5-methylcytosine levels in a microgram of DNA. The effects of chemical carcinogens on 5-methylcytosine levels in micro-systems, tissue biopsies, and human epithelial cells can now be studied. These studies have also shown that the expression of hepatitis B core antigen and human gammaglobin are correlated with the loss of specific methylation sites in the respective promoter regions of these genes. Human tumor DNAs will, therefore, be probed for DNA methylation pattern alterations in selective DNA sequences and genes. Since chemical carcinogens have been shown to decrease genomic 5-methylcytosine levels in BALB/3T3 cells, DNA from carcinogen-treated human epithelial cells will also be probed for changes in 5-methylcytosine patterns.